Marine seismic surveys utilize acoustic waves transmitted to the earth's crust and reflected back to recording sensors. The recording sensors may be hydrophones and/or geophones in one of a number of streamers that may be towed behind a survey boat. The survey boat may tow an array of multiple streamers in parallel. Multiple sources for transmitting acoustic waves may also be used in marine seismic surveys.
Various types of sources may be used for conducting marine seismic surveys. One type of source is an air gun, or an array of air guns. During seismic surveys using air guns, an air gun may fire high-pressure air into the water, creating a bubble. The bubble may initially expand. As the bubble expands, the air pressure within the bubble decreases. When the air pressure within the bubble falls below the hydrostatic pressure from the surrounding water, the air bubble will then collapse. As the bubble collapses, the air pressure therein increases, causing the bubble to eventually expand again. Thus, the bubble will oscillate at an oscillation period that depends on the initial air pressure of the bubble, the volume of air released from the air gun, and the hydrostatic pressure of the surrounding water. The oscillation of the bubble further results in acoustic waves transmitted to the water to the bottom of the sea. The acoustic waves may in part penetrate the sea bottom. The acoustic waves may be reflected back by subterranean interfaces upward where they may be detected by hydrophones and geophones. Data produced from detected waves may reveal formations in the Earth's crust below the sea bottom.
The seismic energy source is actuated at a selected depth in the water. About half of the energy travels downwards where it passes through the water bottom and into the rock formations. The remainder of the acoustic energy travels upwards from the air gun or air gun array, and most of this energy reflects from the water surface whereupon it is 180-degree phase shifted and thereafter travels downwards. The reflected energy from the water surface will be delayed in time and phase shifted relative to the directly downward propagating acoustic energy. The surface-reflected, downwardly traveling acoustic energy is commonly known as a “ghost” signal. The ghost signal interferes with the directly downward propagating wave-field causing constructive interference in some parts of the frequency band and destructive interference in other parts of the frequency band. This causes a sequence of notches in the spectrum, approximately equally spaced in frequency, including a notch at zero frequency (0 Hz). The frequencies of these notches in the detected seismic signal are related to the depth at which the air gun or air gun array is disposed, as is well known in the art. The effect of the reflected energy from the sea surface is typically referred to as the “source ghost”.
To avoid deep notches in the spectrum and reduce the effects caused by the source ghost, seismic surveys such as those described above may be conducted with multiple sources positioned at different depths. Various techniques for conducting seismic surveys using multiple sources have been developed. Such techniques may rely on different configurations having multiple sources positioned at different depths.
While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and description thereto are not intended to be limiting to the particular form disclosed, but, on the contrary, is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.